Thin-film transistor having a plurality of island-like regions

ABSTRACT

A thin-film transistor comprising an active silicon pattern which is formed of a plurality of island-like regions arranged in parallel to each other, the island-like regions being formed of a polycrystal silicon thin film and each having a plane area of 1000 μm 2  or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor formed of a polycrystalsilicon thin film, which is formed by crystallizing an amorphous siliconthin film on an insulator, and a thin-film transistor using thatsemiconductor.

2. Description of the Related Art

In recent years, a technique has been popularly researched where anamorphous silicon thin film is formed on an insulator such as a quartzsubstrate, allowed to grow into a solid-phase crystal (SPC) through aheating treatment or an annealing process due to the irradiation of alaser beam or an intense light to form a polycrystal silicon thin film.

A conventional general technique for obtaining a polycrystal siliconthin-film by allowing an amorphous silicon thin film to grow in a solidphase will be described below.

First, an amorphous silicon thin film having a thickness of 500 to 5000Å is formed on a quartz substrate.

Thereafter, the amorphous silicon thin film thus formed is heated to400° to 1100° C. and subjected to an annealing process to make theamorphous silicon thin film grow into crystal. In this situation, aheater, infrared rays or the like is used as heating means.

The annealing process may be conducted by the irradiation of a laserbeam or an intense light other than heating.

In the above-mentioned manner, a polycrystal silicon thin film isobtained.

The polycrystal silicon thin film thus obtained is used as an activesilicon layer of the thin-film transistor (TFT), to thereby provide athin-film transistor. As a result, a liquid-crystal display unit, imagesensor or the like which is high in operation speed and in image qualityis obtained using the above thin film transistor.

Up to now, the polycrystal silicon thin film obtained by annealing theamorphous silicon thin film has difficulty in lowering a defect densityin crystal.

The thin-film transistor using the polycrystal silicon thin film thusformed as the active silicon film is hindered from realizing animprovement in various characteristics of the thin-film transistor, forexample, the lowering of a threshold voltage (V_(th)), an increase inmobility, a decrease in a leak current (I_(off)), etc., because thedefect density in the active silicon layer is high.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances,and therefore an object of this invention is to make a polycrystalsilicon thin film obtained by annealing an amorphous silicon thin filmlow in defect density and high in quality.

Another object of the invention is to provide a thin-film transistorusing a polycrystal silicon thin film obtained through an annealingprocess with a lowered threshold voltage (V_(th)) and leak current(I_(off)) and an increased mobility.

In order to solve the above-mentioned problem, according to one aspectof the present invention, there is provided a method of manufacturing asemiconductor characterized in that, in polycrystallizing an amorphoussilicon thin film formed on a substrate through an annealing process,said amorphous silicon thin film has a plane area of 1000 μm² or less.

In the above-mentioned method, the amorphous silicon thin film ispreferably 1000 Å or more, more preferably 2000 Å to 10000 Å inthickness.

Also, according to another aspect of the present invention, there isprovided a thin-film transistor having an active silicon film which isformed of a plurality of island-like regions arranged in parallel toeach other, said island-like regions being formed of a polycrystalsilicon thin film where each of the island-like regions has a plane areaof 1000 μm² or less.

In the thin-film transistor thus constituted, the island-like regionsare formed of a polycrystal silicon thin film which is preferably 1000 Åor more, more preferably 2000 Å to 10000 Å in thickness.

Also, according to another aspect of the present invention, there isprovided a method of manufacturing a thin-film transistor comprising thesteps of: forming an amorphous silicon thin film on a substrate;processing said amorphous silicon thin film into a plurality ofisland-like regions each of which has a plane area of 1000 μm² or less;polycrystallizing an amorphous silicon thin film that forms saidisland-like regions through an annealing process; and forming athin-film transistor having at least one of said plurality ofisland-like regions as an active silicon layer.

In the above-mentioned method, the amorphous silicon thin film ispreferably 1000 Å or more, more preferably 2000 Å to 10000 Å inthickness.

The present applicant has found that after the amorphous silicon thinfilm has been formed as island-like regions each having a plane area (anarea viewed from an upper surface of the substrate) of 1000 μm² or less,it is subjected to an annealing process by heating or the irradiation ofa laser light beam or an intense light to form a polycrystal siliconthin film with the result that a polycrystal silicon thin film which islow in defect density and high in quality is obtained.

FIG. 1 is a graph showing a relationship between a threshold voltage(V_(th)) and an area of the island-like regions of the polycrystalsilicon thin-film transistor, in which each of the island-like regionsis 1250 Å in thickness.

As shown in FIG. 1, it has been found that as the area of theisland-like regions is small, the threshold voltage approaches 0 (zero)more in both of a p-channel and an n-channel so that the defect densityis lowered.

In FIG. 1, there has been found that an extremely excellent crystallineproperty is obtained when the plane area of the island-like regions iseach 1000 μm² or less.

Also, when the plane area of the island-like regions is each 1000 μm² orless, the island-like regions may be of a square, a rectangle or othershapes.

Further, when the island-like regions are 1 μm² or more in plane area,the semiconductor is satisfactorily available as a device, and also itcan be readily manufactured by a usual technique.

On the other hand, in the case where the polycrystal silicon thin filmis provided as the active silicon layer of the thin-film transistor,because an area of the island-like regions is limited in size, thethin-film transistor using that polycrystal thin film is also limited insize, which causes a limit to the performance of the thin-filmtransistor.

Under the above circumstances, the applicant has found that a pluralityof island-like regions a plane area of each of which is 1000 μm² or lessare arranged in parallel to each other as active silicon layers thatconstitute the source region, the drain region and the channel formationregion of a thin-film transistor so as to increase a substantial channelwidth, thereby being capable of obtaining a polycrystal thin-filmtransistor which allows a sufficient amount of current to flow therein,which has the channel formation region low in defect density, and whichis high in performance.

FIG. 3 shows an example of a plane shape of a thin-film transistor usinga plurality of island-like regions as an active silicon layer.

In FIG. 3, a plurality of island-like regions 301 are arranged inparallel to each other to form an active silicon layer 305 of athin-film transistor. On the island-like regions 301 are disposed a gateelectrode 302, a source electrode 303 and a drain electrode 304 to formone thin-film transistor. Appropriate intervals between the respectiveisland regions is several to several tens μm. As the intervals aresmall, the plane area of the active silicon layer can be reduced more.

In the island-like region, as its plane area is reduced, the defectdensity is reduced more in a polycrystallized state, thereby beingcapable of reducing a leak current.

Also, the applicant has found that the amorphous silicon thin film isset to preferably 1000 Å or more, more preferably 2000 Å to 10000 Å inthickness, thereby lowering the defect density of the polycrystalsilicon thin film obtained by crystallizing that amorphous silicon thinfilm.

FIG. 2 is a graph showing a relationship between the electron spindensity of a polycrystal silicon thin film in a solid-phase growth andthe thickness of an initial amorphous silicon thin film. In this case, atemperature of a solid-phase crystal (SPC) is 600° C. The electron spindensity is to a certain extent inversely proportional to the defectdensity.

It has been found from FIG. 2 that as the thickness is made thin, thedefect density is reduced more (the electron spin density rises).

However, in crystallizing the thick initial amorphous silicon thin filmthrough an annealing process, a stress of about 3×10⁻⁹ dyn/cm² due to aphase change is developed with the result that the polycrystal siliconthin film formed may be cracked.

Accordingly, when the polycrystal silicon thin film formed bycrystallizing the thin amorphous silicon thin film is used as an activesilicon layer that constitutes the channel formation region of thethin-film transistor as it is, this may cause the defect of the deviceor the lowering of the performance.

However, the applicant has found that even though the thickness of theamorphous silicon thin film is 1000 Å or more, more particularly 2000 Åto 10000 Å, if area of each of the the island-like regions formed of theamorphous silicon thin film is set to 1000 μm² or less in area and thenannealed for crystallization, a polycrystal silicon thin film isobtained which has no cracks and is lower in the defect density.

Also, when the thickness of the amorphous silicon thin film becomesthicker than 10000 Å, cracks are liable to occur.

According to the present invention, there can be obtained a polycrystalthin-film transistor that allows a sufficient amount of current to flowtherein, has a channel formation region which is low in defect density,and is high in performance.

The thin-film transistor like this can reduce power consumption becausea threshold voltage (V_(th)) and a leak current (I_(off)) are lowered.Also, the thin-film transistor can operate at a high speed and allow alarge current to flow therein because the mobility (μ) is increased.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbe more apparent from the following description taken in conjunctionwith the accompanying drawings.

FIG. 1 is a graph showing a relationship between a threshold voltage(V_(th)) and an area of island-like regions of a polycrystal siliconthin-film transistor;

FIG. 2 is a graph showing a relationship between the electron spindensity of a polycrystal silicon thin film in a solid-phase growth andthe thickness of an initial amorphous silicon thin film;

FIG. 3 is a diagram showing an example of a plane shape of a thin-filmtransistor using a plurality of island-like regions as an active siliconlayer;

FIGS. 4A to 4D are diagrams showing a process of manufacturing asemiconductor in accordance with a first embodiment; and

FIGS. 5A to 5D are diagrams showing the upper surfaces of thesemiconductor shown in FIGS. 4A to 4D.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, a description will be given in detail of embodiments of the presentinvention.

(First Embodiment)

A first embodiment shows an example in which an active matrix circuitand a peripheral drive circuit both of which are formed of polycrystalsilicon thin-film transistors are formed on the same substrate.

FIGS. 4A to 4D show a process of manufacturing the semiconductor inaccordance with the first embodiment.

FIGS. 5A to 5D show the upper surfaces of the semiconductor incorrespondence with FIGS. 4A to 4D, respectively. FIGS. 5A to 5D arediagrams viewed from the upper portions of FIGS. 4A to 4D. FIGS. 4A to4D are cross-sectional views taken along a line A--A' of FIGS. 5A to 5D,respectively.

In FIGS. 4A to 4D, quartz was first used for a substrate 401. Instead, aglass substrate such as Corning Corp. 7059 may be used.

The substrate 401 is cleaned, and a silicon oxide under film 402 havinga thickness of 2000 Å is formed through the plasma CVD technique usingTEOS (tetra ethoxy silane) and oxygen as a raw gas.

Then, an initial amorphous silicon thin film having a thickness of 1000Å or more, preferably 2000 Å to 10000 Å, in this example 3000 Å isformed by the plasma CVD technique.

Subsequently, the initial amorphous silicon thin film is patternedthrough the dry etching in such a manner that island-like regionsforming active silicon layers 403 to 405 are disposed at positions wherethin-film transistors of an active matrix portion and a peripheral drivecircuit portion are formed (FIG. 4A).

As shown in FIG. 5A, there are formed island-like regions 501 to 507formed of amorphous silicon thin films to thereby form active siliconlayers 403 to 405.

The size of the respective island-like regions is 20 μm×50 μm(width×length) in this example so that the area of the plane shape ofeach island is set to 1000 μm² or less.

In the peripheral drive circuit portion requiring a high-speed drive,three island-like regions are provided for one thin-film transistor, andin the active matrix portion demanding a reduced leak current, oneisland-like region is provided for one thin-film transistor. It isneedless to say that the number of island-like regions may be increasedin accordance with a required standard.

In this example, the respective intervals between the adjacentisland-like regions that form one thin-film transistor of the peripheraldrive circuit portion was set to 4 μm.

Also, in the thin-film transistor of the active matrix portion, theactive silicon layer 405 is formed by one island-like region in thisexample, however, it is needless to say that it may be formed by aplurality of island-like regions.

Also, the active silicon layer 405 may be formed by a plurality ofisland-like regions each having a smaller plane area. In this case, thedefect density is lowered more so that the leak current can be lowered.

Further, the shape of the island-like regions forming a thin-filmtransistor may be different between the active matrix portion and theperipheral drive circuit portion.

Then, the island-like regions formed of those amorphous silicon thinfilms are crystallized through the annealing process.

A substrate temperature was set to 500° C. to 1100° C., in this example700° C., and a heating time was set to 2 hours to 72 hours, in thisexample 48 hours.

The annealing process may be conducted by the irradiation of a laserbeam or an intense light (infrared rays or the like) other than heating.

Through that crystallizing process, the island-like regions 501 to 507were formed into a polycrystal silicon thin film which has beencrystallized excellently.

Thereafter, through the plasma CVD technique, a silicon oxide film 406that functions as a gate insulating film was formed at a thickness of1500 Å.

On the silicon oxide film 406, an aluminum film 6000 Å in thickness isformed through the sputtering technique and then patterned by etching,thereby forming gate electrodes 407 to 409.

Subsequently, the active silicon layers 403 to 405 were doped withimpurities giving an n-type conduction type or a p-type conduction typein a self-matching manner with a mask of the gate electrodes 407 to 409through the ion doping technique.

In this example, as the doping gas, phosphine (PH₃) was used for then-type doping, and diborane (B₂ H₆) was for the p-type doping.

In this example, the thin-film transistor in the pixel region was madethe p-channel type. In other words, the active silicon layers 404 and405 were doped with the p-type impurities, and the active silicon layer403 was doped with the n-type impurities.

As a result, there can be formed p-type impurity regions 413, 415, 416and 418, n-type impurity regions 410 and 412, andsubstantially-intrinsic channel formation regions 411, 414 and 417.

Thereafter, an annealing process was conducted on those regions at 400°C. to 800° C. for 1 to 12 hours, typically at 600° C. for two hours sothat the doped impurities are activated (FIG. 4B).

Shown in FIG. 5B is that in the respective active silicon layers 403 and404, the gate electrodes 407 and 408 are disposed on a plurality ofisland-like regions.

Then, an insulating film consisting of a silicon nitride 500 Å inthickness and a silicon oxide 3000 Å in thickness was formed as a firstinterlayer insulator 419 thereon through the plasma CVD technique.

Subsequently, contact holes 420 to 424 are formed in the firstinterlayer insulator 419, and electrodes/wirings 425 to 428 were formedusing a multi-layer film made of metal material, for example, titanium500 Å in thickness and aluminum 4000 Å in thickness (FIGS. 4C and 5C).

In the first embodiment, one of the contact holes 420 to 423 of theactive silicon layers 403 and 404 is formed for three island-likeregions as shown in FIG. 5C. However, one contact hole may be formed foreach island-like region.

Thereafter, a silicon oxide thin film 4000 Å in thickness is furtherformed thereon through the plasma CVD technique as a second interlayerinsulator 429.

Then, a contact hole 430 was formed in the impurity region at the activematrix region side where the pixel electrode of a thin-film transistoris formed, and an ITO (indium tin oxide) film 800 Å in thickness wasformed thereon. That film was so etched as to form a pixel electrode 431(FIGS. 4D and 5D).

In that way, the active matrix portion and the peripheral drive circuitportion can be formed on the same substrate.

The active matrix circuit and the peripheral drive circuit thus formedis excellent because it is small in leak current (I_(off)), low in powerconsumption, and high in operation speed.

That substrate and an opposing substrate having an electrode on onesurface thereof are located through liquid crystal, thereby beingcapable of manufacturing a liquid-crystal electro-optical device.

As was described above, according to the present invention, apolycrystal thin-film transistor can be obtained which allows asufficient amount of current to flow therein, which has the channelformation region low in defect density, and which is high inperformance.

The thin-film transistor like this allows the threshold voltage (V_(th))and the leak current (I_(off)) to be lowered, the power consumption canbe lowered. Also, because the mobility (μ) is increased, the thin-filmtransistor can operate at a high speed and allow a large current to flowtherein.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the invention. Theembodiment was chosen and described in order to explain the principlesof the invention and its practical application to enable one skilled inthe art to utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto, and their equivalents.

What is claimed is:
 1. A thin-film transistor comprising:an activesilicon pattern which is formed of a plurality of island-like regionsarranged in parallel to each other, each of said island-like regionsbeing formed of a polycrystal silicon film having a planar area of 1000μm² or less.
 2. The transistor of claim 1 wherein each of saidisland-like regions is formed of a polycrystal silicon film which is1000 Å or more in thickness.
 3. The transistor of claim 1 wherein eachof said island-like regions is formed of a polycrystal silicon filmwhich is 2000 Å to 10000 Å in thickness.